Predicting the number of lifetime divisions for hematopoietic stem cells from telomere length measurements

How many times does a typical hematopoietic stem cell (HSC) divide to maintain a daily production of over 1011 blood cells over a human lifetime? It has been predicted that relatively few, slowly dividing HSCs occupy the top of the hematopoietic hierarchy. However, tracking HSCs directly is extremely challenging due to their rarity. Here, we utilize previously published data documenting the loss of telomeric DNA repeats in granulocytes, to draw inferences about HSC division rates, the timing of major changes in those rates, as well as lifetime division totals. Our method uses segmented regression to identify the best candidate representations of the telomere length data. Our method predicts that, on average, an HSC divides 56 times over an 85-year lifespan (with lower and upper bounds of 36 and 120, respectively), with half of these divisions during the first 24 years of life.

Chromosome-Length Haplotypes with StrandPhaseR and Strand-seq

Dense local haplotypes can now readily be extracted from long-read or droplet-based sequence data. However, these methods struggle to combine subchromosomal haplotype blocks into global chromosome-length haplotypes. Strand-seq is a single cell sequencing technique that uses read orientation to capture sparse global phase information by sequencing only one of two DNA strands for each parental homolog. In combination with dense local haplotypes from other technologies, Strand-seq data can be used to obtain complete chromosome-length phase information. In this chapter, we run the R package StrandPhaseR to phase SNVs using publicly available sequence data for sample HG005 of the Genome in a Bottle project.

Parent-of-origin detection and chromosome-scale haplotyping using long-read DNA methylation sequencing and Strand-seq

Hundreds of loci in human genomes have alleles that are methylated differentially according to their parent of origin. These imprinted loci generally show little variation across tissues, individuals, and populations. We show that such loci can be used to distinguish the maternal and paternal homologs for all human autosomes without the need for the parental DNA. We integrate methylation-detecting nanopore sequencing with the long-range phase information in Strand-seq data to determine the parent of origin of chromosome-length haplotypes for both DNA sequence and DNA methylation in five trios with diverse genetic backgrounds. The parent of origin was correctly inferred for all autosomes with an average mismatch error rate of 0.31% for SNVs and 1.89% for insertions or deletions (indels). Because our method can determine whether an inherited disease allele originated from the mother or the father, we predict that it will improve the diagnosis and management of many genetic diseases.

Telomeres, Telomerase and Cancer

Telomeres and telomerase play a crucial role in human aging and cancer. Three "drivers" of human aging can be identified. The developmental program encoded in DNA is the primary determinant of lifespan. Faithful execution of the developmental program requires stability of the (epi-)genome which is challenged throughout life by damage to DNA as well as epigenetic 'scars' from error-free DNA repair and stochastic errors made during the establishment and maintenance of the "epigenome". Over time (epi-)mutations accumulate, compromising cellular function and causing (pre-)malignant alterations. Damage to the genome and epigenome can be considered the second "driver" of aging. A third driver of the aging process, important to suppress tumors in long-lived animals, is caused by progressive loss of telomeric DNA. Telomere erosion protects against cancer early in life but limits cell renewal late in life, in agreement with the Antagonistic Pleiotropy theory on the evolutionary origin of aging. Malignant tumors arise when mutations and/or epimutations in cells (clock 2) corrupt the developmental program (clock 1) as well as tumor suppression by telomere erosion (clock 3). In cancer cells clock 3 is typically inactivated by loss of p53 as well as increased expression of telomerase. Taken together, aging in humans can be described by the ticking of three clocks: the clock that directs development, the accumulation of (epi-)mutations over time and the telomere clock that limits the number of cell divisions in normal stem and immune cells.

A survey of current methods to detect and genotype inversions

Polymorphic inversions are ubiquitous in humans, and they have been linked to both adaptation and disease. Following their discovery in Drosophila more than a century ago, inversions have proved to be more elusive than other structural variants. A wide variety of methods for the detection and genotyping of inversions have recently been developed: multiple techniques based on selective amplification by PCR, short- and long-read sequencing approaches, principal component analysis of small variant haplotypes, template strand sequencing, optical mapping, and various genome assembly methods. Many methods apply complex wet lab protocols or increasingly refined bioinformatic analyses. This review is an attempt to provide a practical summary and comparison of the methods that are in current use, with a focus on metrics such as the maximum size of segmental duplications at inversion breakpoints that each method can tolerate, the size range of inversions that they recover, their throughput, and whether the locations of putative inversions must be known beforehand. This article is protected by copyright. All rights reserved.

Sex differences in telomere length, lifespan, and embryonic dyskerin levels

Telomerase levels in most human cells are insufficient to prevent loss of telomeric DNA with each replication cycle. The resulting “Hayflick” limit may have allowed lifespan to increase by suppressing the development of tumors early in life be it at the expense of compromised cellular responses late in life. At any given age, the average telomere length in leukocytes shows considerably variation between individuals with females having, on average, longer telomeres than males. Sex differences in average telomere length are already present at birth and correspond to reported differences in the average life expectancy between the sexes. Levels of telomerase RNA and dyskerin, encoded by DKC1, are known to limit telomerase activity in embryonic stem cells. X‐linked DKC1 is expressed from both alleles in female embryo cells and higher levels of dyskerin and telomerase could elongate telomeres prior to embryo implantation. The hypothesis that embryonic telomerase levels set the stage for the sex differences in telomere length and lifespan deserves further study.

Mapping of sister chromatid exchange events and genome alterations in single cells

Mammalian genomes encode over a hundred different helicases, many of which are implicated in the repair of DNA lesions by acting on DNA structures arising during DNA replication, recombination or transcription. Defining the in vivo substrates of such DNA helicases is a major challenge given the large number of helicases in the genome, the breadth of potential substrates in the genome and the degree of genetic pleiotropy among DNA helicases in resolving diverse substrates. Helicases such as WRN, BLM and RECQL5 are implicated in the resolution of error-free recombination events known as sister chromatid exchange events (SCEs). Single cell Strand-seq can be used to map the genomic location of individual SCEs at a resolution that exceeds that of classical cytogenetic techniques by several orders of magnitude. By mapping the genomic locations of SCEs in the absence of different helicases, it should in principle be possible to infer the substrate specificity of specific helicases. Here we describe how the genome can be interrogated for such DNA repair events using single-cell template strand sequencing (Strand-seq) and bioinformatic tools. SCEs and copy-number alterations were mapped to genomic locations at kilobase resolution in haploid KBM7 cells. Strategies, possibilities, and limitations of Strand-seq to study helicase function are illustrated using these cells before and after CRISPR/Cas9 knock out of WRN, BLM and/or RECQL5.

Construction of Strand-seq libraries in open nanoliter arrays

Single-cell Strand-seq generates directional genomic information to study DNA repair, assemble genomes, and map structural variation onto chromosome-length haplotypes. We report a nanoliter-volume, one-pot (OP) Strand-seq library preparation protocol in which reagents are added cumulatively, DNA purification steps are avoided, and enzymes are inactivated with a thermolabile protease. OP-Strand-seq libraries capture 10%-25% of the genome from a single-cell with reduced costs and increased throughput.

Deposition Bias of Chromatin Proteins Inverts under DNA Replication Stress Conditions

Following DNA replication, equal amounts of chromatin proteins are distributed over sister chromatids by re-deposition of parental chromatin proteins and deposition of newly synthesized chromatin proteins. Molecular mechanisms balancing the allocation of new and old chromatin proteins remain largely unknown. Here, we studied the genome-wide distribution of new chromatin proteins relative to parental DNA template strands and replication initiation zones using the double-click-seq. Under control conditions, new chromatin proteins were preferentially found on DNA replicated by the lagging strand machinery. Strikingly, replication stress induced by hydroxyurea or curaxin treatment and inhibition of ataxia telangiectasia and Rad3-related protein (ATR) or p53 inactivation inverted the observed chromatin protein deposition bias to the strand replicated by the leading strand polymerase in line with previously reported effects on replication protein A occupancy. We propose that asymmetric deposition of newly synthesized chromatin proteins onto sister chromatids reflects differences in the processivity of leading and lagging strand synthesis.

InvertypeR: Bayesian inversion genotyping with Strand-seq data

Background

Single cell Strand-seq is a unique tool for the discovery and phasing of genomic inversions. Conventional methods to discover inversions with Strand-seq data are blind to known inversion locations, limiting their statistical power for the detection of inversions smaller than 10 Kb. Moreover, the methods rely on manual inspection to separate false and true positives.

Results

Here we describe “InvertypeR”, a method based on a Bayesian binomial model that genotypes inversions using fixed genomic coordinates. We validated InvertypeR by re-genotyping inversions reported for three trios by the Human Genome Structural Variation Consortium. Although 6.3% of the family inversion genotypes in the original study showed Mendelian discordance, this was reduced to 0.5% using InvertypeR. By applying InvertypeR to published inversion coordinates and predicted inversion hotspots (n = 3701), as well as coordinates from conventional inversion discovery, we furthermore genotyped 66 inversions not previously reported for the three trios.

Conclusions

InvertypeR discovers, genotypes, and phases inversions without relying on manual inspection. For greater accessibility, results are presented as phased chromosome ideograms with inversions linked to Strand-seq data in the genome browser. InvertypeR increases the power of Strand-seq for studies on the role of inversions in phenotypic variation, genome instability, and human disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07892-9.

Fully phased human genome assembly without parental data using single-cell strand sequencing and long reads

Human genomes are typically assembled as consensus sequences that lack information on parental haplotypes. Here we describe a reference-free workflow for diploid de novo genome assembly that combines the chromosome-wide phasing and scaffolding capabilities of single-cell strand sequencing1,2 with continuous long-read or high-fidelity3 sequencing data. Employing this strategy, we produced a completely phased de novo genome assembly for each haplotype of an individual of Puerto Rican descent (HG00733) in the absence of parental data. The assemblies are accurate (quality value > 40) and highly contiguous (contig N50 > 23 Mbp) with low switch error rates (0.17%), providing fully phased single-nucleotide variants, indels and structural variants. A comparison of Oxford Nanopore Technologies and Pacific Biosciences phased assemblies identified 154 regions that are preferential sites of contig breaks, irrespective of sequencing technology or phasing algorithms.

breakpointR: an R/Bioconductor package to localize strand state changes in Strand-seq data

MOTIVATION

Strand-seq is a specialized single-cell DNA sequencing technique centered around the directionality of single-stranded DNA. Computational tools for Strand-seq analyses must capture the strand-specific information embedded in these data.

RESULTS

Here we introduce breakpointR, an R/Bioconductor package specifically tailored to process and interpret single-cell strand-specific sequencing data obtained from Strand-seq. We developed breakpointR to detect local changes in strand directionality of aligned Strand-seq data, to enable fine-mapping of sister chromatid exchanges, germline inversion and to support global haplotype assembly. Given the broad spectrum of Strand-seq applications we expect breakpointR to be an important addition to currently available tools and extend the accessibility of this novel sequencing technique.

AVAILABILITY AND IMPLEMENTATION

R/Bioconductor package https://bioconductor.org/packages/breakpointR.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Detection of Circulating Tumor Cells in the Diagnostic Leukapheresis Product of Non-Small-Cell Lung Cancer Patients Comparing CellSearch® and ISET

Circulating tumor cells (CTCs) detected by CellSearch are prognostic in non-small-cell lung cancer (NSCLC), but rarely found. CTCs can be extracted from the blood together with mononuclear cell populations by diagnostic leukapheresis (DLA), therefore concentrating them. However, CellSearch can only process limited DLA volumes (≈2 mL). Therefore, we established a protocol to enumerate CTCs in DLA products with Isolation by SizE of Tumor cells (ISET), and compared CTC counts between CellSearch® and ISET. DLA was performed in NSCLC patients who started a new therapy. With an adapted protocol, ISET could process 10 mL of DLA. CellSearch detected CTCs in a volume equaling 2 × 108 leukocytes (mean 2 mL). CTC counts per mL were compared. Furthermore, the live cell protocol of ISET was tested in eight patients. ISET successfully processed all DLA products-16 with the fixed cell protocol and 8 with the live cell protocol. In total, 10-20 mL of DLA was processed. ISET detected CTCs in 88% (14/16), compared to 69% (11/16, p < 0.05) with CellSearch. ISET also detected higher number of CTCs (ISET median CTC/mL = 4, interquartile range [IQR] = 2-6, CellSearch median CTC/mL = 0.9, IQR = 0-1.8, p < 0.01). Cells positive for the epithelial cell adhesion molecule (EpCAM+) per mL were detected in similar counts by both methods. Eight patients were processed with the live cell protocol. All had EpCAM+, CD45-, CD235- cells isolated by fluorescence-activated cell sorting (FACS). Overall, ISET processed larger volumes and detected higher CTC counts compared to CellSearch. EpCAM+ CTCs were detected in comparable rates.

Sperm DNA damage causes genomic instability in early embryonic development

Genomic instability is common in human embryos, but the underlying causes are largely unknown. Here, we examined the consequences of sperm DNA damage on the embryonic genome by single-cell whole-genome sequencing of individual blastomeres from bovine embryos produced with sperm damaged by γ-radiation. Sperm DNA damage primarily leads to fragmentation of the paternal chromosomes followed by random distribution of the chromosomal fragments over the two sister cells in the first cell division. An unexpected secondary effect of sperm DNA damage is the induction of direct unequal cleavages, which include the poorly understood heterogoneic cell divisions. As a result, chaotic mosaicism is common in embryos derived from fertilizations with damaged sperm. The mosaic aneuploidies, uniparental disomies, and de novo structural variation induced by sperm DNA damage may compromise fertility and lead to rare congenital disorders when embryos escape developmental arrest.

Analysis of Released Circulating Tumor Cells During Surgery for Non-Small Cell Lung Cancer

PURPOSE

Tumor cells from patients with lung cancer are expelled from the primary tumor into the blood, but difficult to detect in the peripheral circulation. We studied the release of circulating tumor cells (CTCs) during surgery to test the hypothesis that CTC counts are influenced by hemodynamic changes (caused by surgical approach) and manipulation.

EXPERIMENTAL DESIGN

Patients undergoing video-assisted thoracic surgery (VATS) or open surgery for (suspected) primary lung cancer were included. Blood samples were taken before surgery (T0) from the radial artery (RA), from both the RA and pulmonary vein (PV) when the PV was located (T1) and when either the pulmonary artery (T2 open) or the PV (T2 VATS) was dissected. The CTCs were enumerated using the CellSearch system. Single-cell whole-genome sequencing was performed on isolated CTCs for aneuploidy.

RESULTS

CTCs were detected in 58 of 138 samples (42%) of 31 patients. CTCs were more often detected in the PV (70%) compared with the RA (22%, P < 0.01) and in higher counts (P < 0.01). After surgery, the RA but not the PV showed less often CTCs (P = 0.02). Type of surgery did not influence CTC release. Only six of 496 isolated CTCs showed aneuploidy, despite matched primary tumor tissue being aneuploid. Euploid so-called CTCs had a different morphology than aneuploid.

CONCLUSIONS

CTCs defined by CellSearch were identified more often and in higher numbers in the PV compared with the RA, suggesting central clearance. The majority of cells in the PV were normal epithelial cells and outnumbered CTCs. Release of CTCs was not influenced by surgical approach.

Ongoing chromosomal instability and karyotype evolution in human colorectal cancer organoids

Chromosome segregation errors cause aneuploidy and genomic heterogeneity, which are hallmarks of cancer in humans. A persistent high frequency of these errors (chromosomal instability (CIN)) is predicted to profoundly impact tumor evolution and therapy response. It is unknown, however, how prevalent CIN is in human tumors. Using three-dimensional live-cell imaging of patient-derived tumor organoids (tumor PDOs), we show that CIN is widespread in colorectal carcinomas regardless of background genetic alterations, including microsatellite instability. Cell-fate tracking showed that, although mitotic errors are frequently followed by cell death, some tumor PDOs are largely insensitive to mitotic errors. Single-cell karyotype sequencing confirmed heterogeneity of copy number alterations in tumor PDOs and showed that monoclonal lines evolved novel karyotypes over time in vitro. We conclude that ongoing CIN is common in colorectal cancer organoids, and propose that CIN levels and the tolerance for mitotic errors shape aneuploidy landscapes and karyotype heterogeneity.

Abstract PR13: TERT is frequently mutated in adult-type granulosa cell tumors of the ovary compared to other malignant sex cord-stromal tumors

The telomerase reverse transcriptase (TERT) gene is highly expressed in stem cells and silenced upon differentiation. Cancer cells can attain immortality by activating TERT to maintain telomere length and telomerase activity, a crucial step of tumorigenesis. Two somatic mutations in the TERT promoter (C228T; C250T) have been identified in multiple cancers, such as melanoma and glioblastoma, as gain-of-function mutations that promote transcriptional activation of TERT. A recent study investigating TERT promoter mutations in ovarian carcinomas found mutations in 15% of clear cell carcinomas. However, it is unknown whether these mutations are prevalent in adult-type granulosa cell tumors (AGCTs) of the ovary, Sertoli-Leydig cell tumors (SLCTs), and other malignant sex cord-stromal tumors.

We performed whole-genome sequencing on ten AGCT cases with matched normal and identified the TERT C228T promoter mutation in 50% of cases. We found that AGCT cases with mutated TERT promoter have increased expression of TERT mRNA compared to those with wild-type TERT promoter. All five TERT promoter mutated cases had high levels of TERT mRNA expression, whereas three of the five wild-type TERT cases had no measurable TERT mRNA expression. There was a tendency towards longer telomere lengths in AGCT cases with the TERT promoter mutation relative to those without, although it was not statistically significant. These results suggest that telomerase may be activated by a different method in the cases with no TERT promoter mutations but have TERT mRNA expression. The remaining cases with neither TERT promoter mutations nor TERT mRNA expression likely maintain their telomeres using a telomerase-independent method, such as the alternative lengthening of telomeres pathway. TERT C228T allelic discrimination analysis of 331 AGCTs, 5 SLCTs, and 18 other malignant sex cord-stromal tumors detected the mutation in 56/247 (23%) of primary AGCTs, 22/84 (26%) of recurrent AGCTs, 1/5 (20%) of SLCTs and (0/18) 0% of other malignant sex cord-stromal tumors. The single SLCT case with the TERT promoter mutation was poorly differentiated and harbored the pathognomonic FOXL2 mutation of AGCT, suggesting this SLCT case may actually be an AGCT. In 204 AGCT cases with available survival data, there was a trend towards worse disease-specific survival in patients with the TERT promoter mutation compared to those without; however, statistical significance was not reached (p = 0.128, log-ranked test). In 5 AGCT cases with primary and recurrent tissues, we found that the TERT promoter mutation was absent in the primary tumors but present in the recurrent tumors, suggesting that TERT C228T mutation may play an active role in progression of AGCTs.

Overall, we found that TERT C228T promoter mutation was most common in AGCTs among the different malignant sex cord-stromal tumors. Our data confirm the activation of telomerase in AGCTs via TERT C228T promoter mutation, although alternative telomerase activation methods in AGCTs may exist. Our results suggest that TERT activation may play a role in AGCT recurrence. As such, telomere biology may be important for the progression of AGCTs.

This abstract is also being presented as Poster B54.

Citation Format: Jessica A. Pilsworth, Dawn R. Cochrane, Zhouchunyang Xia, Geraldine Aubert, Anniina E. M. Färkkilä, Hugo M. Horlings, Satoshi Yanagida, Winnie Yang, Jamie L. P. Lim, Yikan Wang, Ali Bashashati, Jacqueline Keul, Adele Wong, Esther Oliva, Sohrab P. Shah, Stefan Kommoss, Friedrich Kommoss, Peter M. Lansdorp, Duncan M. Baird, David G. Huntsman. TERT is frequently mutated in adult-type granulosa cell tumors of the ovary compared to other malignant sex cord-stromal tumors. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr PR13.

Assembling draft genomes using contiBAIT

Summary

Massively parallel sequencing is now widely used, but data interpretation is only as good as the reference assembly to which it is aligned. While the number of reference assemblies has rapidly expanded, most of these remain at intermediate stages of completion, either as scaffold builds, or as chromosome builds (consisting of correctly ordered, but not necessarily correctly oriented scaffolds separated by gaps). Completion of de novo assemblies remains difficult, as regions that are repetitive or hard to sequence prevent the accumulation of larger scaffolds, and create errors such as misorientations and mislocalizations. Thus, complementary methods for determining the orientation and positioning of fragments are important for finishing assemblies. Strand-seq is a method for determining template strand inheritance in single cells, information that can be used to determine relative genomic distance and orientation between scaffolds, and find errors within them. We present contiBAIT, an R/Bioconductor package which uses Strand-seq data to repair and improve existing assemblies.

Availability and Implementation

contiBAIT is available on Bioconductor. Source files available from GitHub.

Contact

koneill@bcgsc.ca or mark.hills@stemcell.com.

Supplementary information

Supplementary data are available at Bioinformatics online.

p53 Prohibits Propagation of Chromosome Segregation Errors that Produce Structural Aneuploidies

The presence of an abnormal karyotype has been shown to be profoundly detrimental at the cellular and organismal levels but is an overt hallmark of cancer. Aneuploidy can lead to p53 activation and thereby prevents proliferation, but the exact trigger for p53 activation has remained controversial. Here, we have used a system to induce aneuploidy in untransformed human cells to explore how cells deal with different segregation errors. We show that p53 is activated only in a subset of the cells with altered chromosome content. Importantly, we find that at least a subset of whole-chromosome aneuploidies can be propagated in p53-proficient cells, indicating that aneuploidy does not always lead to activation of p53. Finally, we demonstrate that propagation of structural aneuploidies (gain or loss of part of a chromosome) induced by segregation errors is limited to p53-deficient cells.

BLM helicase suppresses recombination at G-quadruplex motifs in transcribed genes

Bloom syndrome is a cancer predisposition disorder caused by mutations in the BLM helicase gene. Cells from persons with Bloom syndrome exhibit striking genomic instability characterized by excessive sister chromatid exchange events (SCEs). We applied single-cell DNA template strand sequencing (Strand-seq) to map the genomic locations of SCEs. Our results show that in the absence of BLM, SCEs in human and murine cells do not occur randomly throughout the genome but are strikingly enriched at coding regions, specifically at sites of guanine quadruplex (G4) motifs in transcribed genes. We propose that BLM protects against genome instability by suppressing recombination at sites of G4 structures, particularly in transcribed regions of the genome.

Bloom syndrome is characterized by high levels of sister chromatid exchanges (SCEs). Here, the authors use single-cell DNA template strand-sequencing to map SCEs in patient cells, and propose that the BLM helicase protects the genome against unwanted recombination at sites of G-quadruplex structures.

Genome-wide mapping of sister chromatid exchange events in single yeast cells using Strand-seq

Homologous recombination involving sister chromatids is the most accurate, and thus most frequently used, form of recombination-mediated DNA repair. Despite its importance, sister chromatid recombination is not easily studied because it does not result in a change in DNA sequence, making recombination between sister chromatids difficult to detect. We have previously developed a novel DNA template strand sequencing technique, called Strand-seq, that can be used to map sister chromatid exchange (SCE) events genome-wide in single cells. An increase in the rate of SCE is an indicator of elevated recombination activity and of genome instability, which is a hallmark of cancer. In this study, we have adapted Strand-seq to detect SCE in the yeast Saccharomyces cerevisiae. We provide the first quantifiable evidence that most spontaneous SCE events in wild-type cells are not due to the repair of DNA double-strand breaks.

Dense and accurate whole-chromosome haplotyping of individual genomes

The diploid nature of the human genome is neglected in many analyses done today, where a genome is perceived as a set of unphased variants with respect to a reference genome. This lack of haplotype-level analyses can be explained by a lack of methods that can produce dense and accurate chromosome-length haplotypes at reasonable costs. Here we introduce an integrative phasing strategy that combines global, but sparse haplotypes obtained from strand-specific single-cell sequencing (Strand-seq) with dense, yet local, haplotype information available through long-read or linked-read sequencing. We provide comprehensive guidance on the required sequencing depths and reliably assign more than 95% of alleles (NA12878) to their parental haplotypes using as few as 10 Strand-seq libraries in combination with 10-fold coverage PacBio data or, alternatively, 10X Genomics linked-read sequencing data. We conclude that the combination of Strand-seq with different technologies represents an attractive solution to chart the genetic variation of diploid genomes.

Guanine quadruplex monoclonal antibody 1H6 cross-reacts with restrained thymidine-rich single stranded DNA

Previously we reported the production and characterization of monoclonal antibody 1H6 raised against (T₄G₄)₂ intermolecular guanine quadruplex (G4) DNA structures (Henderson A. et al. (2014) Nucleic Acids Res., 42, 860–869; Hoffmann R.F. et al. (2016) Nucleic Acids Res., 44, 152–163). It was shown that 1H6 strongly stains nuclei and has exquisite specificity for heterochromatin by immuno-electron microscopy. Here we extend our studies of 1H6 reactivity using enzyme-linked immunosorbent assay (ELISA) and microscale thermophoresis (MST). As previously reported, 1H6 was found to strongly bind intermolecular G4 structures with a (T₄G₄)₂ sequence motif. However, using both methods we did not detect significant binding to G4 structures without thymidines in their sequence motif or to G4 structures made with (T₂G₄)₂ oligonucleotides. In addition, we observed strong, sequence-specific binding of 1H6 by ELISA to immobilized single stranded poly(T) DNA but not to immobilized poly(C) or poly(A) homo-polymers. Cross-reactivity of 1H6 to poly(T) was not measured in solution using MST. 1H6 was furthermore found to bind to selected areas on DNA fibers but only after DNA denaturation. Based on these observations we propose that 1H6 binds with high affinity to adjacent T's that are restricted in their movement in selected G4 structures and denatured DNA. Cross-reactivity of 1H6 to immobilized single stranded T-rich DNA next to its previously reported specificity for bona fide G4 structures needs to be taken into account in the interpretation of 1H6 binding to (sub-) cellular structures.